[Legate of Amon Lanc]

I can only quote Gandalf here:

Originally Posted by FotR, Chapter 2 "Shadow of the Past"

"Alas! there is little hope of that for him. Yet not no hope. No, not though he possessed the Ring so long, almost as far back as he can remember. For it was long since he had worn it much: in the black darkness it was seldom needed. Certainly he had never "faded". He is thin and tough still. But the thing was eating up his mind, of course, and the torment had become almost unbearable."

I think this is quite clear. Gollum, as much as other halflings, was quite resistant to the effects of the Ring. And he probably was not very much nearer to becoming a Ringwraith than, let's say, Bilbo; or Frodo after the Morgul-wound.